The present invention is related generally to a method of treating a nickel titanium alloy, known as Nitinol, for use in manufacturing instruments having improved resistance to cyclic fatigue failure. As a particular application, the invention is related to preparation of Nitinol wire blanks for use in manufacturing instruments for use in the human body having improved resistance to cyclic fatigue failures.
Many medical applications take advantage of the properties of Nitinol, a nickel and titanium alloy. Nitinol (an acronym for Nickel Titanium Naval Ordinance Laboratory) exhibits several useful properties such as shape memory, by which a Nitinol component returns to a previously memorized shape after being forced into a second shape. Nitinol also exhibits superelasticity, meaning that a Nitinol component may be deformed elastically to a very large extent by strain without reducing its ability to return to its original shape after the strain has been removed. Nitinol is very flexible and resistant to cyclic fatigue when compared to stainless steel, which makes Nitinol the material of choice in medical and dental applications. However, cyclic fatigue remains a common problem with Nitinol medical and dental instruments. For example, Shen at al. compared the incidence and mode of instrument separation of two Nitinol rotary systems, ProFile and ProTaper. A total of 166 ProFile and 325 ProTaper instruments—which were used according to a predefined schedule of clinical use by the same group of operators—was analyzed. Flexural fatigue was implicated in the majority of separations in both groups, with 66% of the separated ProFile instruments and 95% of the ProTaper instruments fracturing because of cyclic fatigue. See Ya Shen et al., Comparison of Defects in ProFile and ProTaper Systems after Clinical Use, 32 J. Endodontics 61 (No. 1, January 2006).
As would be understood by those of skill in the art, Nitinol alloys can exist in one of two different temperature-dependent crystal structures—austenite at higher temperatures and martensite at lower temperatures—and within a given temperature range, the alloy can stabilize as one or the other. In general terms this temperature-dependent phase transformation is from martensite to austenite during heating, while the reverse transformation from austenite to martensite starts upon cooling. As the temperature increases above a certain critical temperature, known as the austenite start temperature or AS, the alloy rapidly changes in composition between the martensite and austenite form and completes the transition to austenite at a critical temperature known as the austenite finish temperature or AF.
There are two known methods for obtaining a target AF Temperature: varying the nickel-to-titanium ratio and thermally heat treating the material at its finished form. The AF of Nitinol is affected directly by the ratio of nickel to titanium during production of the ingot. The target AF temperature can be lowered or raised by varying the nickel percentage alone. The AF temperature is also directly affected by the processing of the material post-ingot form by the amount of cold work, the temperatures at which the material is thermally heat treated, and the amount of strain induced during processing. Therefore, a target AF is achievable by varying the nickel percentage, the thermal heat treat process of the material, or both accordingly.
Because Nitinol in the martensite form is soft and malleable, it demonstrates improved fatigue resistance, which is due to the differing crystalline structures between Nitinol in its martensitic and austenitic state. Austenitic Nitinol's crystalline structure is known to be that of a face centered cubic lattice, while the martensitic crystalline structure is that of a monoclinic distorted structure with atomic dislocations.
The distorted structure of martensitic Nitinol allows for the material to be deformed at greater angles and working conditions than that of the austenitic Nitinol in those same angles or working conditions. Generally, when the working or working environment is above the AF temperature, meaning the Nitonol is in its austenitic state, the material cannot withstand the same level of stress/strain—such as cyclical fatigue—as it can withstand when it is below the AF temperature and transitioning to (or transitioned into) its martensitic state. Nitinol in the austenite form is strong and hard, having a much more regular crystalline lattice structure and exhibiting properties similar to those of titanium. In the field of endodontic instruments, conventional wisdom holds that Nitinol must be primarily in its austenitic state in order to provide required stiffness and strength.
Nitinol alloys can also undergo a phase change between austenite and martensite as a result of the application of a strain. Therefore, a strain-induced martensite can exist in the alloy at temperatures up to a martensitic deformation temperature MD, which is typically above the austenite finish temperature AF. For example, Nitinol in the austenitic phase can be bent so that at high strain locations the alloy becomes martensitic. If the alloy is designed to have an unstable martensite phase at its intended application's operating temperature, removal of the strain results in a reverse transformation that straightens the bending. This reverse transformation is an example of what is known as shape memory and is considered an essential feature of Nitinol.
The prior art teaches a process by which Nitinol in its martensitic state undergoes cold and hot cycling to stabilize the martensitic twins and therefore significantly improve fatigue performance (see U.S. Pat. No. 7,648,599 to Berendt, titled “Method of Preparing Nickel Titanium Alloy for Use in Manufacturing Instruments with Improved Fatigue Resistance.”) This cycling process works to stabilize the crystalline structure of the Nitinol in its more martensitic condition and maintains the stabilized martensitic twins for long term usage.
Subsequent testing has shown that this hot and cold cycling is not necessary to achieve a level of improved fatigue performance sufficient for medical and dental instruments, which typically are single use instruments. In other words, it is not necessary to provide the stabilized martensitic twins because the fatigue resistance provided by the twins is well beyond the amount of cycling encountered during a medical and dental instrument's single use. Further, it is not necessary to provide the shape memory feature. Shape memory is not a desired feature in instruments rotating around a curve or being placed in a curve because the shape memory is a restorative force. As the instrument attempts to straighten itself, it can damage the surrounding tissue (e.g., an arterial wall or a tooth root canal).
Last, in the process of finishing a Nitinol medical instrument, machining operations such as grinding may be employed. These machining operations degrade the physical properties of the material. For example, prior to machining operations a Nitinol wire prepared using the method of U.S. Pat. No. 7,648,599 might be five-times more resistant to cyclic fatigue than Nitinol wires prepared by conventional means. After machining operations, this same wire might be only three-times more resistant. The machining processes produce mechanical stress and frictional heat which alter the characteristics of the surface of wire.